1. Field of the Invention
The present invention relates to nitride films, and particularly methods to reduce the formation of cracks in gallium nitride films for semiconductor devices.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by reference numbers enclosed in brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below at the end of the Detailed Description of the Preferred Embodiment. Each of these publications is incorporated by reference herein.)
The deposition of GaN films on silicon substrates is difficult because of a large thermal expansion coefficient mismatch between the two materials. Most deposition techniques involve the deposition of buffer layers or stress-relief layers with a distinct composition from that of the substrate and that of GaN; there is an abrupt composition variation between the buffer layer and the GaN layer. These techniques result in GaN films which are under tensile stress at room temperature. Tensile stress favors the formation of macroscopic cracks in the GaN, which are detrimental to devices fabricated thereon.
GaN and its alloys with InN and AlN are used in visible or UV light-emitting devices (e.g. blue laser diodes) as well as high-power, high-frequency electronic devices (e.g. field-effect transistors). Because of the lack of GaN substrates, such devices are typically fabricated from a thin layer of GaN deposited on a substrate such as sapphire (Al2O3) or silicon carbide (SiC). Although both substrates are available in single-crystal form, their lattice constant is different than that of GaN. This lattice mismatch causes extended defects such as dislocations and stacking faults to be generated at the interface between the substrate and the GaN layer as well as into the GaN layer itself. The use of buffer layers such as AlN or low-temperature GaN and the optimization of deposition conditions typically yields films with approximately 109 threading dislocations per square centimeter. More novel techniques such as lateral epitaxial overgrowth (LEO), “pendeoepitaxy,” and maskless LEO result in lower dislocation densities (as low as 106 cm−2).
Although GaN-based devices are currently being mass-produced using both sapphire and silicon carbide substrates, the use of silicon substrates is expected to bring about further cost reductions as well as improvement to the capability of those devices. For example, silicon can be etched using simple chemicals, which allows simple substrate-removal techniques to be utilized with GaN-based films or devices. Silicon is also the material on which most of the electronic devices (e.g. microprocessors) have been developed; integrating GaN-based devices with silicon-based electronic functions would create new types of systems. Silicon is readily available in large wafer sizes with excellent crystal quality at low cost, such that devices grown on silicon may be less expensive than equivalent devices grown on sapphire or silicon carbide. Finally, silicon is a better thermal conductor than sapphire.
The growth of GaN on silicon substrates presents similar challenges as on sapphire and silicon carbide. The lattice mismatch between the (001) plane of GaN and the (111) plane of silicon is 17.6%, compared to 16% for sapphire and 3.5% for silicon carbide. The use of a thin AlN buffer has yielded GaN films on Si(111) with as low as 3×109 threading dislocations per square centimeter. However, the thermal expansion mismatch of GaN with silicon is +31%, compared to −26% for sapphire and +17% for silicon carbide. (The positive sign indicates a thermal expansion coefficient larger for GaN than for the substrate.) Assuming for the sake of demonstration that the GaN film is stress-free at the growth temperature (typically 1000 degrees centigrade), a positive thermal expansion mismatch would result in a GaN film under tensile stress after cool-down to room temperature. GaN films exhibit cracking when the tensile stress exceeds approximately 400 MPa. Cracks generally render devices inoperable due to electrical shorts or open circuits. In general the stress associated with the lattice mismatch, including any relaxation effect that may occur during growth, is referred to as “grown-in stress”. The stress arising from the thermal expansion mismatch when the film is cooled from the growth temperature to room temperature is referred to as “thermal stress”. The sum of the grown-in stress and thermal stress is the net stress in the film.
Several methods of forming GaN films on silicon substrates have been suggested. Takeuchi et al. [1] propose a buffer layer composed of at least aluminum and nitrogen, followed by a (GaxAl1-x)1-yInyN layer. Based on technical papers published by the same group (e.g. [2], [3]) the resulting films are under tensile stress, as can be assessed by photoluminescence spectroscopy measurements. The films exhibit cracking. Extensive work at the University of California, Santa Barbara (UCSB) resulted in significant improvements in crystal quality using this method; however the GaN films were always found to be under tensile stress (200-1000 MPa), which usually caused cracking. Takeuchi et al. [4] also propose 3C—SiC as a buffer layer. The resulting GaN films also exhibit cracking, which is strong evidence that they are under tensile stress. Yuri et al. [5] propose an extension of this method wherein the silicon substrate is chemically etched after the deposition of a thin layer of GaN on the SiC buffer layer, such that subsequent deposition of GaN is made possible without the tensile stress problems, associated with the presence of the silicon substrate. Marx et al. [6] propose the use of GaAs as an intermediate layer. Shakuda [7] proposes a method of forming GaN-based light-emitting devices on silicon wafers on which a silicon nitride (Si3N4) layer has been deposited.
In all the aforementioned techniques, there is a finite composition step between the substrate and the buffer layer as well as between the buffer layer and the GaN layer. The difference in composition is associated with a difference in lattice constants which, in general, means that a certain amount of elastic energy is present in the layers. The elastic energy is stored in the form of compressive strain if the (unstrained) lattice constant of the top layer is larger than that of the bottom layer. The elastic energy is maximized if the top layer grows pseudomorphically on the bottom layer, that is, if the top layer adopts the in-plane lattice constant of the bottom layer. For the cases under discussion the amount of elastic energy may exceed the energy required to form defects such as islands or dislocations, which reduce the energy of the strained layer. This is especially true if the growth is interrupted, because in general growth interruptions allow a coherently strained layer to evolve into islands. In this case the elastic energy stored in the top layer is reduced compared to the pseudomorphic case.
There is a need for methods of reducing the formation of cracks in gallium nitride films for semiconductor devices. Accordingly, there is also a need for such methods to produce compressive, rather than tensile, stresses in the films. There is further a need for methods to produce such films on common substrates such as silicon. The present invention meets these needs.